I'm building a marshall head from scratch. Now, I've got all the soldering pretty much done but when I came to finish the grounding bus I got confused. I emailed with the amp guru from London Power Press (tube amp guy in Canada) and got more confused!

So I just need to get all these grounds connected. This is my first amp and I don't want to fry everything or kill myself :) because I grounded it wrong.

What about the ground on the input jacks? Do they ground to the same star as the preamp stage?

KISS

13th January 2007 01:12 AM

It's a good idea to use isolated input jacks and connect the "ground" of the input jack through a resistor 1-10ohms to the star point. This will tend to allow a place for ground currents to go and help reduce hum.

Sometimes it make sense to create a "signal ground". This ground esentially does not carry current. Connect all signal grounds together.

Then create a "power ground" and connect all grounds that return current.

Chassis ground should be a "protective ground" and connected to earth. There shouldn't be a bunch of connections to the chassis.

You can then connect power and signal together with or with or without a resistor. Protective ground is always connected to chassis.

A "Star" is the best topology, but chassis ground should be separated.

Grounding is very often mis-understood.

bastieng

15th January 2007 01:06 AM

I'm not sure what I ground the preamp star to. Do I solder a wire from the preamp ground to the power star? Second thing, what do I ground the power transformer to? From what I've ready I ground the PT centertap, output tubes, and the two power filter caps to the first "power star". What do I ground that star to?

So, in your case the preamp power supply is dirty, so it belongs in the power star. Your connection to from power star to earth may or may not be required.

Now, I'll throw in a curve, but no one seems to design this way.

1a) The dirty ground (1) goes to neutral of the power line.

2a) The 0 volt reference goes connectes to 1 and 3
at the load center of the house

3a) The outer case goes to (3). (3) is always connected to Neutral at the load center of the house (assuming no sub-panels).

So, what you have in the ideal case is that (1), (2) and (3) are connected to EARTH at the load center of the house which is the 0 volt reference of the entire house and faults don't disturb the reference.

(3) carries current only in a fault condition.

bastieng

15th January 2007 07:06 PM

In the 50W plexi 1987 circuit there are 6 power supply caps(3 dual cap cans). They are more or less in a chain. The last in the chain feeds the preamp phase. I read you should use the minus post on that cap as the star ground for the preamp phase.

Does that sound right?

KISS

16th January 2007 09:09 PM

If it's a coupling cap, then yes. If it's part of the high pass filter, then yes. What's the value of this cap?

There are much better caps to use in this application compared to yesteryear. I was working on an X-ray power supply (100 KV, 0.1A) that was made in the 40's to 50's. I changed the arc supression caps to one specifically made for that duty, not polarized paper caps. These caps were operated on the AC mains volatge and were rated at 600 VDC. They should have been AC rated caps, but they didn't exist back then. They were visibly leaking. X-ray supply is back in business except for the formalities: alignment, test run and being blessed by our safety dept.

If it's power to the preamp then No. Attach to dirty ground.

A lot of the can caps of yesteryear had the can at the ground potential and connected to the chassis. There were caps that separated the "CAN" and they typically had a paper outside and were not conncetd to chassis.

Where is this cap located? A general location like in the audio path to the grid or the B+ to the preamp stage to "ground/common"?

bastieng

19th January 2007 07:09 PM

These caps are on top of the amp, outside of the chassis. This is typical for a plexi layout. Two are by the power transformer and one is closer to the preamp end.